SaveService: output JMX from JMeterTreeNode or JMeterTreeModel

**Wyatt Epp** ([Bug 60336](https://bz.apache.org/bugzilla//show_bug.cgi?id=60336&redirect=false)):
This is something that would be useful to have if you're working with the API.

I specifically found need for it when working with the ProxyControl API because deliverSampler works with JMeterTreeNodes. So to output usable JMX with SaveService, I had to write code that traverses a JMeterTreeModel and builds a ListedHashTree. (It didn't end up being especially long, but it was tricky to debug.)

OS: All